Hi all,

I'm new to the group and 've been trying to get a good crater glaze with big crater holes and it's driving me crazy because all my test results look like bubbly grey oatmeal with no actual craters. I've tried incrementally adding silicon carbide and a bunch of other suggested things, but to no avail.

I'm hoping for a possible recipe to test at cone 6.
I'm a fan James Lovera's work and love the big crater holes and textures in glazes.
Thanks in advance for any help you can give!
-- Susanne

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Both Cryolite and Silicon Carbide tend to be fully reacted by the time a Cone 6 glaze starts to set up as it cools. So the typical result is pretty sad looking, like this Cone 6 "Crater Glaze" with crylolite. Craters?

What works is a glaze like Marilee Lava which contains 24% calcium carbonate, with just a homeopathic amount of silicon carbide 0.3% - I can only assume this minute amount of silicon carbide is added as a good luck charm, although I've never left it out. I personally believe the glaze is very viscous and most of the gas is produced by the calcium carbonate, but that's just a guess.

This is what it looks like at our studio, even with our 6 hour slow-cool between 1,800 F and 1,500 F.  Notice the cobalt is a flux as well as a colorant, like copper, so the cobalt-colored glaze retains more of the bubbles than the Mason Stain colored "lava" as these colors are refractory.

Without a slow-cool, more of the bubbles are preserved and it probably looks like this photo I copied from someplace, likely from Marilee.

You'll notice the surface of the lava glaze on this cup is suspiciously flat and smooth with the bubbles cut open, as if a grinder had been used to make the surface uniform - and I'd be very surprised if that were not the case.

I suppose it could have been fired in a silica plaster mold, but that's not the easiest solution.


The recipe from our Insight-Live database.

Thanks I'll try that one and let you know!

Thank you, Norm Stuart, for such a thorough and informative response (as always).  I'm not even looking for a crater glaze and I appreciate this response!  Bonus points for use of the phrase "homeopathic amount."

Slightly related: I had some interesting results years ago from cryolite in the following recipe.  I hope it's not irresponsible to post this, as the results were volatile and fragile.  The cryolite spits a little in the kiln and the resulting wrinkled surface could be popped with your fingers.  That said, I share it only because it was one of the most beautiful surfaces I've ever seen – rich, vibrant blue with pronounced wrinkling, not bubbling.  It's worth tinkering with to stabilize, if it's possible.

62%  Cryolite

12%  Frit 3110

2.5%  Alumina Hydrate 

0.5%  Cobalt Carbonate (This may well have been cobalt oxide given how rich the color was and how small the amount is.)

Susanne Scher said:

Thanks I'll try that one and let you know!

Thank you! I have been getting a lot of info from potters I've reached out to. I'll list some of the suggestions here in case they are helpful, although I have not begun to test them all yet. I'll be sure to post any good results I get for the tests. I appreciate all the suggestions!


?? Anyone know what the glaze chemical "Cady Cal" is? One of the older recipes (below) calls for it, and no one I have asked has ever heard of it!


I love Josh Herman's work, he gets those nice big crater holes and drips in vivid colors.


I even emailed him and was pleased to get a responce to add a LOT more Silicon Carbide like 20% instead of 2%. Also some have said to fire at cone 5 instead of cone 6, which is hard for me to do since my other glazes mature at cone 6.


I also found some interesting effects with recipes at glazy.org. This one looks interesting but I have not tested it yet.



Ti Phillips (from: www.earthstokenfire.com) suggests this recipe and instruction:

Thank you for your recent question. Crater glazes are also known as volcanic or froth glazes. Although these are commonly described as defects, a glaze purposely created for this type of "defect" is no longer considered a defect to the glaze surface, but a decorative surface. Lucy Rie purposely created glazes to achieve this type of effect, but there are other potters who find the froth surface a unique and beautiful design, if done correctly.

The key to a froth or volcanic glaze is silicon carbide or barium sulphate. These two ingredients create a bubbling effect under the glaze. When fired, the kiln must be turned off and temperature brought down to a dull red at a specific time in order to achieve the effect. Although commonly performed on low fired ceramics, this process can be achieved on high fire, if you take the time to watch your kiln.

To create the volcanic reaction, create a slip using the same clay body your pot is made from. For every 100 grams of wet slip, add 10 grams of silicon carbide or barium sulphate. Sieve into a 300 mesh sieve. Mix and apply where you wish the froth to take place.

Use the recipe below and glaze your pot as normal. The recipe will create a buttery glaze matt at cone 9 (need to adjust somehow for cone 6 I guess)

    Custer Feldspar 50

    Dolomite 20

    Cadycal 100 5  (WHAT IS THIS??  and what does it mean to list 100 then 5?)

    EPK Kaolin 25


    Vanadium Oxide 5


Jackie Masters has some great lava glazes that are reminiscent of Lucie Rie and I also reached out to her. She says: " I fire to 1300, so I have no idea how you would achieve the crater holes you are looking for at cone 6.  I am still working on the crater, volcanic glazes and don’t feel I will ever be finished.  All I can suggest is trial and error.  It has taken over 15 year for me to get the results I get now and even they are not set as the basic glaze ingredients are always changing, I guess that is half the fun."


So it's back to testing, testing, testing for me!

Thanks again everyone!!

-- Susanne

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